Electrical wave filter system



Feb. 17, 1942.

Added Photocell Current Brightness.

Final Current. Currents.

J. .c. WILSON 2,273,163

ELECTRICAL WAVE FILTER SYSTEM Filed Aug. 15, 1940 2 Sheets-Sheet 2 +1 r-Widih of Reading Slit.

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INVENTOR HN 0. WILSON ATTORN EY Patented Feb. 17, 1942 ELECTRICAL WAVE FILTER SYSTEM Wilson, Bayside, N. Y., assignor to John C.

Hazeltine Corporation, a corporation of Delaware Application August 15, 1940, Serial No. 352,805

14 Claims.

This invention relates to electrical wave filter systems having approximately a predetermined amplitude-frequency characteristic and a sub-.

stantially linear phase-frequency characteristic and relates particularly to such networks having an amplitude-frequency characteristic related to that of a scanning aperture or scanning spot operating at a predetermined velocity.

It is frequently desirable in wave-signal translatlng systems to providea wave filter system having a predetermined amplitude-frequency characteristic over a given frequency range in .order to compensate for amplitude distortion introduced in some other part, of the system'or' to effect an amplitude distortion of thetranslated 'signal for some other purpose. 1 that, for distortionless transmission of a wave signal, one requisite is that the over-all phase- It is well known frequency characteristic of the translating system-must be linear. However, in general, it has not been possible to construct a network having certain desirable amplitude-frequency characteristics over a range of frequencies which also has a linear phase-frequency characteristic over the same range of frequencies. Accordingly, it has been the practice, in those cases where a network with such a predetermined amplitude-frequency characteristic is required, to provide a network having the desired amplitude-frequency characteristic, but which does not have a linear phase-frequency characteristic, and also to include in cascade therewith some type of phasecorrecting network with a uniform amplitudefrequency characteristic; that is, the required amplitude-frequency characteristic is provided by a network which does not have a linear phase- -frequency characteristic, and some other netaperture introduces no phase distortion into the system. The amplitude distortion for any such aperture operating at a predetermined velocity can be computed in terms of an electrically equivalent amplitude-frequency characteristic and it is frequently desirable to simulate such a characteristic in an electrical system, or to provide a network which is effective to compensate therefor. Furthermore, it is well known that the scanning spot of a television system efiects a distortion analogous to that of an aperture and it will be understood that in the following specification the term aperture characteristic is intended to include also the characteristic of a scanning spot.

The amplitude-frequency characteristic of a scanning aperture of the type under discussion comprises frequencies of zero response, the wave lengths of signals of such frequencies being integral sub-multiples of the aperture width. For a further explanation of these characteristics of a scanning aperture, see the material under the heading of Finite aperture effects" beginning on page 84 of applicants treatise entitled Television Engineering published by Sir Isaac Pitman and Sons, Ltd, London, 1937.

In the following specification and claims the present invention is described and claimed in terms of an electrical wave filter having a predetermined amplitude-frequency characteristic related to that of a scanning aperture operating at a predetermined velocity. It will be understood, however, that this treatment has been adopted to facilitate the description of the invention and that the wave filter systems of the invention are phase-frequency characteristic over a given frewhich is symmetrical about an axis at right angles to the direction of scanning introduces an amplitude distortion into the system due to the quency range, which characteristic is substantiall identical to or related to that of a scanning aperture operating at a predetermined velocity. It is still another object of the invention to related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprises, a multisection delay network effectively including a plurality of filter sections have a pass band including the given frequency range. The delay network is designed to have a time delay at least as great as, and in the preferred embodiments, a time delay which is an integral multiple of, the period corresponding to the first zero-response frequency of the aperture. The system also includes means for deriving voltages from a plurality of junctions of the filter together with means for adding the derived voltages with predetermined relative magnitudes and polarities which are effectively symmetrical about a given point of the filter to procure the desired amplitude-response characteristic.

For abetter understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Figs. 1 and 2 of the drawings comprise graphs utilized toexplain the general theory of the electrical wave filter system of the invention; Fig. 3 is a circuit diagram of an electrical wave filter system in accordance with the invention for providing an amplitude-frequency characteristic substantially identical to that of a square scanning aperture operating at a predetermined velocity; Fig. 4 of the drawings is a circuit diagram of an electrical wave filter system in accordance with the invention which may provide any predetermined amplitude-frequency characteristic; Fig. 5 illustrates a simplified electrical wave filter system for procuring an amplitude-v frequency characteristic which is compensatory to that of a square scanning aperture operating at a predetermined velocity; while Figs. 6a, 6b, 6c, and 6d comprise a series of graphs utilized to explain the operation of the circuit of Fig. 5.

Reference is made to the curves of Fig. 1 for an introduction to the theory of the present invention. Fig. 1 comprises a graph A which represents a given brightness distribution which is to be scanned with a predetermined velocity by a square aperture of size 1:. Any such brightnessdistribution characteristic can be analyzed into its harmonic components, one of which is shown as a dotted sinusoid B of wave length k1: on the displacement axis. When the wave length ha:

is equal to, or any integral sub-multiple of, the

dimension 3:, corresponding components of the brightness-distribution characteristic are completely elided. This is the case for k=1, k= k=%3, etc. and, due to the constant velocity of scanning, the brightness components so considered con be converted into frequencies to derive the attenuation characteristic of Fig. 2, which is plotted up to the frequency for which kz the frequencies corresponding to k=1, k= /2, etc., being termed zero-response frequencies or simply zero frequencies. The characteristic of Fig. 2 is thus a well-known type associated with a square scanning aperture operating with a predetermined velocity. For apertures which are symmetrical about any line at right angles to the direction of travel, there is no phase distortion introduced by such a scanning aperture. Consequently, a scanning aperture may be said to act as a unique kind of filter having a predetermined amplitude-frequency characteristic and having no phase distortion.

In Fig. 3, there is represented an electrical wave filter system in accordance with the present invention which has an amplitude-frequency characteristic similar to that of Fig. 2, or related to that of a scanning aperture operating at a predetermined velocity, and which has a substantially linear phase-frequency characteristic. This electrical wave filter system comprises a multisection delay network, efiectively including a plurality of nondissipative confluent filter sections having series-inductance arms L and shunt-capacitance arms C. The filter section is thus of the low-pass filter type and is designed to have a pass banal including all the frequencies of the range for which the electrical wave filter system of the invention is to provide a predetermined amplitude-frequency characteristic. Input terminals IO, U and output terminals I2, I 3 are provided for the wave filter system. A terminating resistor R, which is effectively an impedance-matching element, is connected across the end of the filter remote from the input terminals I0, H to prevent reflections in the filter circuit. The delay network is designed to have a time delay at least as, and preferably equal to or equal to an integral multiple of, great as the period of the first zero-response frequency of the aperture the characteristic of which is to be approximated by that of the filter.

In order to derive voltages from a plurality of junctions of the filter associated with a portion of the filter having a time delay substantially equal to the period of the first zero-response frequency of the related aperture, there are provided vacuum tubes I5-l9, inclusive, having input elecamplitude-frequency characteristic. A unidirectional operating voltage is supplied for the gacuum tubes i5-i 9, inclusive, by means of source In considering the operation of the electrical wave filter system just described, it will be seen that, if a signal is applied to input terminals l0, II a distorted output signal is derived from the output terminals l2, l3. Letus suppose that the magnification factor of each of vacuum tubes l5-l9, inclusive, in conjunction with the common load resistor 20 is such that, if the signal voltages at all the tapping points on the delay network, 1. e., at all the tube grids, were exactly in phase, the output voltage from terminals 12, I3 would be equal. in magnitude to the input voltage at terminals I0, I I. This means that, if n is the total number of tappings of the delay network, the magnification of each tube is effectively 1/11.. Let T be the total delay of the network in seconds between the first and last tapping points. Then as it approaches an indefinitely large number, T being held constant, the distortion effected by the filter becomes indistinguishable from that eflected by a rectangular scanning aperture moving at a predetermined velocity parallel to one side.

For a pure sinusoidal input of angular frequency w, the'output, in phase and magnitude relative to the voltage at the central tapping point, for an odd number of tapping points, is

given by:

ain wt a a sin w(td) sin w(t2 i)+ etc.

+-Z-sin w(t+d)+%sin w(t+2d)+ etc.

== sin col-kzg sin wt.

- (cos 'wd+cos 2wd+cos i iwd-i-etc.) where; i q

a is the amplitude of the input sinusoid,

d is the tir'nedelay in seconds between one tapping point and the next, to is the angular'frequency, and t is timeln seconds.

Now, (n-1)d=T, so that we may write:

' d=T/(n-l) 'Hence, the output voltage may be written in where =wT/(1l-1). This can be reduced, using a well-known relation (see "Fouriers Series and SphericalHarmonics, by W. E. Byerly, page 32,

section 20) .to the form:

I v a SlIl not (1) When n becomes large, this approaches:

' sin wT/2 sin arf where represents frequency. This is obviously the same type of attenuation factor as that of a rectangular aperture for which the first zero frequency. fz. is equal to UT. On putting. f=1/T, for example, the above Expression 2' vanishes, as it should. i

Expression 1 gives the output voltage for a filter network of the kind shown in Fig. 3 for a finite number of tappings, in terms of the number n of tapping points. Although the above reasoning was carried through-for a filter having a central tapping point, i. e., having an odd number of tappings, closely similar expressions can readily be derived when n is even. The final result, for a large number of tapping points, is precisely the same whether n is odd or even.

It is thus seen that in the circuit of Fig. 3, means including tubes -19, inclusive, are provided for deriving and adding voltages from the filter which are efiectively symmetrical about the an electrical wave filter system which may be designed to have any predetermined amplitudefrequency characteristic and may, for instance, be utilized to provide an amplitude-frequency characteristic which is complementary or compensatory to that of the filter network of Fig. 3. The filter network of Fig. 4 comprises a multisection delay network similar to that of Fig. 3 and similar circuit elements have identical reference numerals. The delay network of Fig. 4 is designed to have a time delay which is substantially equal to the period corresponding to the first zero-response frequency of a related scanning aperture. In order to obtain a predetermined amplitude-frequency characteristic from the system of Fig. 4, there is provided means for deriving the fundamental component of a periodic signal-input wave having a period equal to the time delay of the network, together with means for obtaining certain of the harmonicfrequency components thereof and for combining the derived components with predetermined relative magnitudes and polarities. Specifically,

means are provided for obtaining the funda-f mental, the second, third, and fourth harmonic; components of the applied signal, together with means for adding the derived components with such relative magnitudes and polarities as eifece' tively to procure the desired amplitude-frequency characteristic. Only these harmonic components are utilized in the arrangement of Fig. 4, because such a system becomes quite complicated if correction is made for many harmonic components.

In order to derive the even harmonic components, the signal applied to the input termin'als In, H and a signal derived from the mid-junction 25 of the filter are combined, duplex combining amplifier tube 28 being provided for this purpose. Thus, the tube 26 has one control electrode coupled to terminal l0 and another control electrode coupled to the mid-junction 25 of the filter so that even harmonic components of the applied signal appear across the cathode load resistor 28 of the tube without a reversal of polarity. The even harmonic components of the applied signal are also developed by a duplex combining amplifier tube 30 to which is applied a signal from a junction 29 of the filter displaced I one-fourth of the length of the filter from the terminals M, II and a signal from junction 32 of the filter displaced three-fourth of the length of the filter from terminals I0, I I. These signals are combined in tube 30 to develop a voltage across a cathode load resistor 3! thereof which includes only the even harmonic components of .the applied signal, specifically including the tube.

In order to develop the second harmonic com an input electrode of vacuum tube 35 and the signal developed across cathode load resistor 28 and containing only the second and fourth harmonic components of the applied signal is applied to another input electrode thereof. There is thus produced across the cathode load resistor 40 of tube 36 signals including only the second harmonic component of the applied signals.

In order to develop the third harmonic component of the signal input to the delay network,

the system includes a cathode load combining amplifier 4| to which are applied voltages from junctions of the filter displaced one-third of the length of the delay network; specifically, voltages derived from terminal l0, junction 42, and junction 43 are individually applied to separate input electrodes of the combining amplifier 4| to develop in its cathode load-resistor.

44 the third harmonic component with the same polarity as it appears in the input signal of the system.

In order to derive the of the translated. signal, the harmonic components thereof, developed as described, are subtracted from the signal input to terminals l0, 8!. An anode load combining amplifier 355 is provided for the purpose of combining the derived harmonic components of the applied signal, the derived second, third, and fourth harmonic components of the signal being individually applied to the input electrodes of the amplifier so that a voltage proportional to the harmonic content of the signal, but of opposite polarity, is developed across an anode load resistor 46 of the amplifier. The voltage developed across resistor 46 is combined in a duplex combining amplifier tube 48 with the signal input to the system derived from terminals M, ii to develop in the load resistor 49 thereof the fundamental component of the applied signal. fundamental component of the applied signal together with each of the harmonic components thereof are thereafter applied to a combining device 50 having output terminals 'l2, l3 from which the signal output. of the electrical wave filter system of the invention is derived. In considering the operation of the system of Fig. 4, it, will be seen that a means has been provided fundamental component The for deriving the fundamental component as well as the second, third, and fourth harmonic components of the signal input to terminals in, ii and for combining these signals in device 5 0 with anydesired relative magnitudes and polarities. The principle used to derive the harmonic components of the signal is analogous to that described in an article-in fPractical Mathematics entitled Section 45. Perry's tabular method, by Nels Johann Lennes, published by the Macmillan Company in 1936. Therefore, it will be seen that any desired amplitude-frequency characteristic can be obtained from the wave filter system of Fig. 4 by combining the harmonic components of the signal input to the system with such relative magnitudes and polarity as to procure the desired'efi'ective amplitudefrequency characteristic. Specifically, the net work of Fig. 4 can be made to provide an amplitude-frequency characteristic which is similar to, or compensatory to that of, Fig. 2, and thus to that of a scanning aperture, simply by combining the harmonic components of the applied signal with the correct magnitudes and polarities to procure the required signal output. More particularly, the signals may be combined to produce an amplitude-frequency characteristic which is geometrically complementary to that of Fig. 2. Therefore, the network of Fig. 4 can be used in an electrical system to compensate for distortion efiected by an aperture having the characteristic illustrated in Fig. 2. It will be seen that the signal translated by the delay network of Fig. i is the same at input terminals it, it and at the resistor R. Therefore, the network of Fig. 4 is one in which voltages are derived from various filter points or junctions and comprises a means for adding the derived voltages with predetermined relative magnitudes and polarities which are effectively symmetrical about the point 25 of the'filter or delay network.

However, in most practical wave filter systems, it is. necessary to correct for many harmonic components of the applied signal so that, in.

utilizing the principles upon which the system of Fig. 4 is based, a very large number of circuitelements are required. The simplified cir cuit of Fig. 5 can also be proportioned to provide an amplitude-frequency characteristic which is compensatory to a given aperture characteristic. Fig. 5 comprises a multisection delay network efiectively comprising a plurality of nondissipative filter sections or half-sections including series-inductance arms L and shunt-capacitance arms C, the delay network being terminated at its end remote from the input terminals W, H by a resistor R to prevent reflection. The circuit is so designed that the network has a time delay substantially twice the period of the first zero-response frequency of the related aperture characteristic, or so that each of the two filter portions connected in cascade has a time delay substantially equal to the period corresponding to the first zero-response frequency of the related aperture. A voltage derived from the mid-junction of the filter is combined in a duplex combining amplifier tube 54 with voltages derived from the end junctions of the filter and combined in a duplex combining amplifier tube 85. The combined voltages derived from the end junctions of the filter and developed across a load resistor till of tube 55 are applied with a polarity opposite that of the mid-junction voltage to an input electrode of tube 543 through a coupling condenser 56, the output terminals l2, it; being-coupled across a load resistor 51 of tube 56. A tap 58 is provided on a resistor 59 in the output circuit of tube 55 and the tap is so adjusted that the amplitude of the signal output derived from resistor 59 and applied to tube 56 is substantially one-fourteenth that of the voltage derived from the mid-junction of the filter and applied to. the other input electrode of the tube 5G.

Reference is made to the curves of Figs. (id-6d,

inclusive, for an explanation of the operation ofthe circuit of Fig. 5, based on its analogy to a related scanning aperture operating at a predetermined velocity.

Fig. 60. represents a brightness-versus-dis placement distribution, encountered, for exam- The electrical wave filter system of i "ever, Runges gguation "(see Rings, Zei ts'hr V fii'r Math 1897, page 206x15 nota'dapijedfto the derivatiq'n of a. mode of (iorre ctmnin @phjysical syst em QLater; this qua n was ipransformed ihbo aifbrm "which indicated now,'1

1y accurate ia mximaticns heing 1? zero-response frequency of such aperture, means for deriving voltages from a plurality of junctions of said filter, and means for adding said derived voltages with predetermined relative magnitudes and polarities which are efiectively symmetrical about a given point of said filter to provide said amplitude-frequency characteristic.

2. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of nondissipative confluent filter sections having a pass band including said frequency range, said, delay network having a time delay at least as great as the period corresponding to the first zero-response frequency of such aperture, means for deriving" voltages from a plurality of junctions of said filter, and means for adding said derived voltages with predetermined relative magnitudes and .polarities which are effectively symmetrical about a given point of said filter to provide said amplitude-frequency characteristic.

3. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of filter sections and an impedance-matching termination and having a pass band including said frequency range, said delay network having a time delay at least as great as the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from a plurality of junctions of said filter, and means for adding said derived voltages with predetermined relative magnitudes and polarities which are effectively symmetrical about a given point of said filter to provide said amplitude-frequency characteristic.

4. An electrical wave filter system having approximately a. predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of low-pass filter sections including series-inductance arms and shuntcapacitance arms and having a pass band insubstantially equal to the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from a plurality of junctions of said filter, and means for adding said derived voltages with relative magnitudes and polarities which are effectively symmetrical about a given point of said filter to provide said amplitude-frequency characteristic.

6. An electrical wave filter system having approximately a predetermined amplitude-fre- P which are effectively symmetrical about a given point of said filter to provide saidamplitudefrequency characteristic.

cluding said frequency range, said delay network having a time delay at least as great as the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from a plurality of junctions of said filter,

' and means for adding said derived voltages with relative magnitudes and polarities which are effectively symmetrical about a given point ofsaid filter to provide said amplitude-frequency characteristic.

5. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and 'a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of filter sections having a pass band including said frequency range, said delay network having a time delay '7. An electrical wave filter system having ap-.

proximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of filter sections having a pass band including said frequency range, said delay network having a time delay at least as great asthe period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from a plurality'of junctions of said filter, and means for adding said derived voltages with substantially equal magnitudes and the same polarity to provide said amplitude-frequency characteristic.

8. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of filter sections having a pass band including said frequency range, said delay network having a time delay substantially equal to the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from a plurality of junctions of said filter which are evenly electricallyspaced and symmetrically disposed about a given point of said filter, and means for adding said derived for deriving voltages from a plurality of junctions of said filter, and means for adding said derived voltages with different relative magnitudes and polarities to provide said amplitudefrequency characteristics.

10. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of filter sections having a pass band including said frequency range, said delay network having a time delay substantially equal to the period corresponding to the first zero response frequency of such aperture, means for deriving voltages from a plurality of juncquency range and each having a time delay substantially equal to the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from each of the junctions of said filter network, and means for adding the voltages which are derived from alternate junctions with opposite polarity and predetermined magnitudes to provide said amplitude-frequency characteristic.

13. An electrical wave filter system having approximately a predetermined amplitude-fretions of said filter, and means for adding said I for deriving voltages from a plurality of junctions of said filter and for adding said derived voltages to derive the fundamental and harmonic components of a signal applied to said filter sysstem, and means for adding said derived fundamental-and harmonic components with different relative magnitudes and polarities to provide the desired amplitude-frequency characteristic.

12. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising a plurality of filter portions in cascade each having a pass band including said frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising two filter portions connected in cascade and having a pass band including said frequency range, each of said portions having a time delay substantially equal to the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from each of the junctions of said filter, and

means for adding the voltages derived from the end junctions of said filter to the voltage derived from the center junction thereof with opposite polarity and with predetermined relative magnitudes to provide said amplitude-frequency characteristic.

14. An electrical wave filter system having approximately a predetermined amplitude-frequency characteristic, related to that of a scanning aperture operating at a predetermined velocity, and a substantially linear phase-frequency characteristic over a given frequency range comprising, a multisection delay network effectively comprising two filter portions connected in cascadeand having a pass band including said frequency range, each of said portions having a time delay substantially equal to the period corresponding to the first zero-response frequency of such aperture, means for deriving voltages from each of the junctions of said filter, and means for adding the voltages derived from the end junctions of said filter to the voltage derived from the center junction thereof with opposite polarity and with a magnitude approximately onefourteenth that of the voltage derived from said mid-junction to provide said amplitude-frequency characteristic.

' JOHN C. WILSON. 

